Cyclic AMP is proposed to act in a negative feedback loop controlling the plasma membrane potential in Neurospora and other eukaryotic microorganisms. This model of homeostatic control will be tested in Neurospora by: 1. Definitively testing whether the plasma membrane potential controls cyclic AMP levels. 2. Using three independent methods (exogenous cyclic AMP and cyclic AMP analogs, pharmacological agents which depress endogenous cyclic AMP levels, and mutants deficient in cyclic AMP synthesis) to test control of the membrane potential by cyclic AMP. 3. Probing several specific aspects of cyclic AMP action, including: a. Its possible action via cyclic AMP-dependent protein kinase (in Neurospora). b. Its possible control of the plasma membrane H ion pump activity. c. Its possible control of phosphoinositide metabolism.